Unlike Variant Chests, when Variant Bookshelves is active you can't make bookshelves with modded wood that doesn't have a custom bookshelf, and you can't use various types of wood at the same time.

Hello! Quark's moved to 1.18 now-- unfortunately, this means that (unless your bug is critical and/or game-breaking), we're closing support for issues filed for versions prior to it (e.g. Quark on 1.16).

If your issue still occurs on 1.18, or if your issue happens to be rather critical (e.g. consistently crashing the game with no other mods present), please leave another note or open a new issue. Thanks for your cooperation!